This invention relates generally to the field of motion sensing instruments and, more particularly, to a path for bleeding excess electric charge from an inertial sensing element.
Inertial instruments often have an electrically isolated sensing element or "forcer" mounted for movement relative to a base in response to external forces. One such instrument is an accelerometer which relies on magnetic interaction between a sensing element and a base to correct for inertial movement of the sensing element.
Isolation of a sensing element from its base sets up capacitive paths which cause electric charge on the element to build up and dissipate in an uncontrolled manner. These effects occur exponentially with a long effective time constant. As a result, electrostatic forces between the sensing element and the base vary over time in a manner unrelated to sensed input, producing an unpredictable time-dependent response known as "drift error". Unlike a constant bias or short-lived transient effects, drift error cannot be eliminated by calibration.
Drift error is especially troublesome in accelerometers which apply corrective forces to a movable sensing element by interaction between a stationary permanent magnet and a coil carried by the sensing element. In such instruments, the sensing element is typically mounted for pivotal movement by hinges which also conduct electricity to the coil. Shorting of the hinges is prevented by insulating pads disposed between the hinges and the sensing element. Such pads are thin and have a high dielectric constant, providing ideal capacitive paths which tend to charge the sensing element over time.
It is therefore desirable in many instances to provide a method for eliminating the effects of electrostatic forces caused by the build-up of excess charge on an inertial sensing element.